1. Field of the Invention
The present invention relates to a control apparatus for intermittently driving an actuator by applying a driving voltage based on a clock pulse while changing a duty ratio of the clock pulse by detecting change amount of an object to be controlled and feeding back it to the control apparatus, and relates to a powder feeder or servo system having the control apparatus therein.
2. Description of Related Art
It is conventionally well-known various control apparatuses which control the time average output based on the operation time per one cycle in the rectangular pulse (duty ratio). For example, it is known an ultrasonic motor utilizing an ultrasonic resonator and a servo system utilizing a DC motor. In the ultrasonic motor, it is used a piezoelectric transducer in which the mechanical deformation of a piezoelectric element caused by electric energy is used to generate mechanical vibration of a vibrator and the output of the ultrasonic motor is changed by changing the duty ratio of the driving voltage.
In case that the ultrasonic resonator in the ultrasonic motor is constructed so as to generate both axial vibration (longitudinal vibration) and bending vibration at the same time, elliptic oscillation is generated in the top end of ultrasonic resonator with the resonance frequency. A pipe is attached to the top end of the resonator, and powder is fed in the pipe, then the powder is moved in the certain direction by the elliptic oscillation, this mechanism can therefore be used in the powder feeder.
The powder feeder will be described with reference to FIG. 10. In FIG. 10, powder amount in the powder feeder is detected by a load cell 11 acting as a load sensor and the detected output (voltage) is amplified by an amplifier 12. The amplified output voltage is fed back to a microcomputer 13, which acts as duty ratio control means, through an A/D converter. In the microcomputer 13, the optimum duty ratio is calculated based on the feedback signal and the clock pulse with the optimum duty ratio is output to a drive circuit 14. The drive circuit 14 outputs the driving voltage controlled by the optimum duty ratio. Thereby, the applying time of driving voltage is controlled, and both feed amount (predetermined feed amount) and actual feed amount of the powder are controlled to a predetermined value (target value).
However, in case that, as mentioned above, the actuator such as the ultrasonic motor or DC motor is driven by using the rectangular pulse, there is a problem that spike noise is produced when the actuator is switched ON/OFF and such spike noise gives bad influence to the output voltage from the load cell 11. For instance, in the above ultrasonic motor, the driving voltage with the resonance frequency applied to the ultrasonic motor is produced by an oscillating circuit shown in FIG. 11, for an driving period (active period). In the oscillating circuit in FIG. 11, so-called push-pull circuit with transistors TR1 and TR2 exists at the primary transside. That is, as shown in FIG. 12(a), both transistors TR1, TR2 operate so that one is ON and the other is OFF or vice versa. At that time, when the transistors TR1, TR2 are switched OFF (that is, when the transistors TR1, TR2 operate so as to shut the current), high voltage is produced at the collector of the transistor TR1, TR2.
In view of the above, since noise produces when one of the transistors TR1, TR2 is switched OFF, the wave form of the driving voltage V.sub.ACT becomes the wave form shown in FIG. 12(b). As a result, it concludes that the noise in the driving voltage V.sub.ACT as disturbance is superposed on the output voltage from the load cell 11, thereby the wave form of the output voltage V.sub.CELL becomes the wave form shown in FIG. 12(c).
Here, as shown in FIG. 12(a), one switching cycle (ON/OFF cycle) T1 of each transistor TR1, TR2 corresponds to one cycle T2 of the driving voltage V.sub.ACT. Therefore, if it is actually used for the powder feeder, the wave form of the driving voltage V.sub.ACT becomes the wave form shown in FIG. 6(b) and the wave form of the output voltage V.sub.CELL becomes the wave form shown in FIG. 6(c).
Thus, if data sampling of the output voltage V.sub.CELL is conducted, for example, at the timing A, B, C . . . shown in FIG. 6(d) when feeding back to the microcomputer 13, the output voltage, on which the noise is superposed, is input as the feedback signal. Due to this, the actuator cannot be precisely controlled and there occurs a problem that accuracy of the control system becomes extremely bad.